Integration of semiconductor lasers to planar optical components, such as waveguides, semiconductor optical amplifiers (SOAs) and detectors, is important for photonic integrated circuit (PIC) applications. Lasers such as semiconductor ridge lasers are useful as elements of PICs because these lasers emit light horizontally, where light can be processed by another element that is formed on the horizontal plane of the substrate of the PIC. When working with these types of PICs it is essential to control reflections from the interfaces between the lasers and the integrated photonic components. With proper design, interface reflections may be used to enhance performance of integrated lasers.
One method is to precisely space gaps between components to coherently enhance or reduce reflections from the interfaces. To do this at least one air gap may be etched in a semiconductor' material near a facet. The air gap may be filled with a dielectric material to reduce diffraction losses. Prior art methods describe the use of resonant and anti-resonant etched gaps used to couple between lasers, SOAs and other lasers, taking advantage of the index discontinuity across air gaps to selectively enhance or reduce reflections across interfaces. A similar process has been demonstrated to create semiconductor lasers that make use of etched gaps in the semiconductor material to enhance reflectivity of the laser mirrors. At near-infrared (NIR) wavelengths, electron beam (e-beam) lithography is frequently required to provide the necessary resolution to define the etch masks used to create the resonant gaps.
A semiconductor Bragg reflector composed of air gaps and semiconductor layers may be etched to increase or decrease the reflectance of a facet. Etching a NIR, first order Bragg reflector requires an e-beam lithography machine that is expensive and slow. Optical lithography machines have sufficient resolution to define etched lasers to form a NIR high order Bragg reflector, but do not have sufficient resolution to define etched layers required to form a first order Bragg reflector and, therefore, cannot maximize optical reflection efficiency. An e-beam process is disclosed in a first article entitled “Edge-Emitting Lasers with Short-Period Semiconductor/Air Distributed Bragg Reflector Mirrors,” by Y. Yuan et al., published in the IEEE Photonics Technology Letters, Vol. 9, No. 7, pp. 881-883, July 1997, a second article, entitled “Edge-Emitting GaInAs—AlGaAs Microlasers,” by E. Hofling et al., published in IEEE Photonics Technology, Vo. 11, No. 8, pp. 943-945, August 1999, and a third article entitled “Continuous Wave Operation of 1.55 μm GaInAsP/InP Laser with Semiconductor/Benzocyclobutene Distributed Bragg Reflector,” by Mothi Madhan et al., published in the Japanese Journal of Applied Physics, Vol. 38, pp. L1240-L1242, Nov. 1, 1999. The third article provides details on filling air gaps with a dielectric. These articles are hereby incorporated by reference into the specification of the present invention.
“A Sub-Micron Capacitive Gap Process for Multiple-Metal-Electrode Lateral Micromechanical Resonators,” Wan That Hsu, et al, Technical Digest, 14th International IEEE Micro Electro Mechanical Conference, January 2001, discloses a process for fabricating a semiconductor having gaps between metal electrodes and a polysilicon resonator resident on the semiconductor. With this method, a sacrificial spacer layer is deposited on a substrate. A polysilicon mechanical resonator is then deposited and etched over the sacrificial layer, during which time portions of the sacrificial layer are removed, and the metal electrodes are formed through electroplating on either side of the resonator. The sacrificial layer is ultimately removed in its entirety. The present invention does not operate in the same manner as this process. The Hsu article is hereby incorporated by reference into the present invention.
“12 μm long edge-emitting quantum-dot laser,” S. Rennon, et al, Electronics Letters, May 2001, discloses a series of mirrors and a central waveguide. Each of the mirrors and the central waveguide are etched. First order Bragg mirrors are patterned by electron-beam lithography on the rear side of the waveguide with air gaps etched between the Bragg gratings. Third order mirrors are etched on the front side of the waveguide. The first order air gaps between Bragg mirrors decrease diffraction loss in the laser (compared to third order air gaps) produced by this method. The present invention is not fabricated in the same manner as the invention of Rennon, et al. Rennon, et al is hereby incorporated by reference into the specification of the present invention.
U.S. patent application Ser. No. 09/412,682, entitled “SACRIFICIAL SPACER FOR INTEGRATED CIRCUIT TRANSISTORS,” discloses a semiconductor integrated circuit with a sacrificial sidewall. Specifically, temporary sidewalls are formed along the side of a gate electrode of a semiconductor. Source/drain regions are then formed on the semiconductor alongside the gate electrode, and the temporary sidewalls are removed, resulting in a space between the gate electrode and the source/drain regions. The present invention does not operate in this manner. U.S. patent application Ser. No. 09/412,682 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,486,025, entitled “METHODS FOR FORMING MEMORY CELL STRUCTURES,” discloses two methods for forming memory cell structures in a semiconductor integrated circuit. One method includes the use of a sacrificial spacer layer formed adjacent to the sidewall of a capacitor of a field effect transistor formed on the semiconductor device. A dielectric layer is then formed alongside the spacer layer, through which a bitline stud layer is formed that is electrically connected to the source/drain regions of the field effect transistor. The sacrificial spacer layer is finally removed from the structure. The present invention operates in a different manner from this process. U.S. Pat. No. 6,486,025 is hereby incorporated by reference into the specification of the present invention.
The methods described above effectively create air gaps in semiconductor structures, however the processes are extremely inefficient as applied to optical devices. Typically several masking and etching steps are required to create both the components, such as waveguides, and the air gaps in optical devices. This can be both time-consuming and costly. What is desirable in the art is to create an efficient, inexpensive method of creating optical semiconductor devices with integrated air gaps to alter mirror reflectance.